1. Field of the Invention
The present invention relates to polymeric compounds with nonlinear optical properties, and more particularly, to organic dye molecular materials and nonlinear optical polymer compounds containing chromophores.
2. Description of the Related Art
Recently, the development of high-speed, high-capacity data transmission devices has increased the need for materials that exhibit nonlinear optical properties and research in this field is being actively performed (Ind. Eng. Chem. Res. 1999, 38, 8-33). These nonlinear optical materials are roughly categorized into inorganic materials such as LiNbO3, KHP, quartz and the like, which have been widely used, and organic materials which become more interesting in the field in recent years along with semiconductor materials. Organic materials are advantageous over inorganic materials in terms of their synthesis and processing procedures and that their physical properties including the processing temperature, refractive index, nonlinear optical coefficient, absorption wavelength and the like can be adjusted according to the requirements. This is the reason why research on the organic material becomes increasing in the field (U.S. Pat. Nos. 5,496,899 and 6,229,047).
Organic materials are classified into crystalline molecules and polymeric substances based on their structure. In the early stage of research, many researchers in a variety of fields were very interested in the crystalline molecule due to its large optical coefficient. However, the crystalline molecule is limited by its unstable nature and difficulties in the crystal growing and processing. To address these limitations, approaches to dispersing organic molecules in polymeric media to develop new materials have been actively made in recent years (Chem. Mater. 1999, 11, 1966-1968). The molecular mobility in a polymeric medium reduces thermal stability and molecular aggregation causes significant optical losses, thereby limiting use of the material.
Meanwhile, to develop materials with desirable processing properties of polymers by incorporating nonlinear organic molecules into polymers is greatly acceptable. Nonlinear polymers are classified into main chains and side chains depending on the way of molecular coupling. Recently, highly ordered macromolecules coupled to be chemically stable, called dendrimer, have been developed and many approaches have been made to increase their applications (U.S. Pat. Nos. 5,659,010, 5,496,899, and 6,001,958).
In another aspect of research, some researchers are involved in developing new materials based on covalently-bonded film formation technique improved from the Languir-Blodegette (LB) film formation technique. Although it is difficult to obtain a thin film having an appropriate thickness for use with this method, the resulting thin film has a structure highly ordered to give a very large nonlinear coefficient.
For main chain polymers, nonlinear molecular chromophores are directly used as molecules to be polymerized so that the synthesis of the main chain polymer is difficult and the resulting polymer has poor three-dimensional orientation efficiency during a poling process. For side chain polymers, chromophores are grafted to the core as side chains. In this case, although it is difficult to chemically react the polymer, the selection of core polymer is flexible and a variety of chromophores can be incorporated. The ordered arrangement of chromophores on the polymer chain as its side chains is also advantageous.
Dendrimers are sterically highly symmetric and exhibit different characteristics according to their functional end groups incorporated. Also, appropriate designing of a dendrimer molecular structure can separate individual chromophores. When excess chromophores over a predetermined amount are incorporated into a polymeric material, aggregation occurs due to chromophore-chromophore electrostatic interactions, thereby resulting in a low nonlinearity and optical scattering loss due to micro domain. Recent studies evidently show that these problems can be eliminated by using dendrimers (Appl. Phys. Lett. 2000, 77(24), 3881-3883).
Basically, the development of highly nonlinear materials needs highly nonlinear dye molecules. According to recent research results, a nonlinear molecule needs a structure capable of partial electron polarization and a large dipolar moment to provide a large nonlinearity. It is advantageous that the nonlinear molecule has strong electron donor and electron acceptor end groups. In addition, the nonlinear molecule should be extended by conjugated double or triple bonds to allow the electrons, i.e., the π-electrons, present between the end groups to move freely. It has been also founded that the nonlinearity effect is increased when such conjugated linkages are stably present on the same plane. Some materials with extended π-linkages to increase their nonlinearity have been reported. As the number of π-linkages increases, the visible light absorption region of the material where electron transition absorption occurs is gradually shifted to a long-wavelength region. This directly affects on the optical loss by absorption, thereby limiting its use as an optical device material (J. Am. Chem. Soc. 1999, 121, 472-473; Chem. Mater. 1999, 11, 1966-1968). As an example, a wavelength converter or an optical amplifier is often used as a light source of a wavelength of about 600-800 nm. Thus, it is necessary to develop materials that are transparent in this wavelength range for the optical communication device.
Meanwhile, the nonlinearity is directly proportional to the chromophore density in a unit volume. As described above, a high chromophore density over a predetermined range results in a reduced nonlinearly. Therefore, there is a need to increase the chromophore density in a polymeric material without this adverse effect.